But the circumstances surrounding the attack have formed a consensus among experts: Russian President Vladimir Putin used the attack to send a deliberate message.

The attack targeted Sergei Skripal, a double agent who passed Russian state secrets to British intelligence in the 1990s and early 2000s. He was pardoned and sent to the UK as part of a spy swap in 2010.

Skripal and his daughter were hospitalized by the attack, and hundreds of residents of the town were warned to wash their clothes to remove risk of exposure to the deadly chemical.

Until Skripal’s poisoning, many thought Novichok was a Cold War fiction, but now experts suggest it functions as a calling card for Putin.

Joshua H. Pollack, the editor of The Nonproliferation Review,tweeted that Novichok was a “signature” chemical weapon that the Kremlin most likely meant as a “declaration of indifference to the suspicions of others,” something that amounted to Putin saying, “We don’t care what you think.”

“Using such an agent just down the road from Porton Down, one of the world’s centres of expertise on CW,” or chemical weapons, “only amplifies the message,” Pollack concluded.

Another expert questioned why attackers would use a distinctly Russian chemical agent if they did not want it to look as if Russia was to blame.

“The stunning thing about this attack is why Novichock when there are other alternatives less provocative and more deniable for Russia,” Vipin Narang, an associate professor of political science at MIT with expertise in weapons of mass destruction,tweeted. “But I suppose that’s precisely the point … Wants everyone to know it’s him.”

The UK can expel Russian diplomats on its soil and impose sanctions, but if the experts are correct that Russia’s government carried out the attack so brazenly, then the UK’s real problem is in Moscow, not an embassy in London.